Gas discharge lamp ballast circuit with frequency modulated pulse control

ABSTRACT

The application relates to a ballast for the pulsed-mode operation of gas discharge lamps (EVG) and a use of a self-excited or externally controlled half or full bridge circuit for the same purpose. An EVG of the mentioned kind has a buffered feed d.c. voltage (U dc , udc) from which the utilized power for the GE lamp or GE lamps can be derived. The load circuit in which the GE lamps are provided has a series-connected choke (L1), if necessary a capacitor (C1) can be connected in parallel to the lamp for ignition purposes. Such a circuit is to be so configured that it is possible to change the characteristic light values, i.e. the light output and the color temperature of the gas discharge lamps (GE). This is to be realized without mechanical choke switching and is achieved in that at least one electronic switching element (S1) or at least one such switching element pair (S1, S2, S3, S4) are provided which supply the choke (L1) and the GE lamp periodically with voltage pulses (u ac ). The switching element pairs and the voltage pulses (u ac ) can be frequency modulated (f1, f2) by way of a control signal (f). This frequency modulation can take place stepwise with alternative frequencies or continuously periodically (sine-like) so that a continuous change in the frequency occurs and no disturbing or standing waves which cause optical flickering form in the GE lamp.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a ballast for the pulsed-mode operation of gasdischarge lamps. More specifically, it concerns the use of aself-excited or externally controlled half or full bridge circuit,likewise for the pulsed mode operation of gas discharge lamps.

2. Description of the Related Art

Conventional electronic ballasts (EVG) serve both to improve brightnesscontrol and to make extended and more sparing ignition of high and lowpressure gas discharge lamps (GE) possible. Such electronic ballastssupply a frequency in the range of 20 kHz to 70 kHz to a series resonantload circuit. By changing the frequency of the output voltage of the EVGthe load current in the series-resonant circuit can be adjustedcontinuously and in a stepless manner. The gas discharge lamp that isarranged in the load circuit and has the load current flowing through itcan thus have its brightness changed in a stepless manner. At the sametime, by the output frequency of the EVG approaching the resonantfrequency of the load circuit when the lamp is not ignited, a highvoltage can be generated at the lamp, to cause the lamp to ignite.

Whilst such EVG's enable comfortable brightness control and ignition,they are not equally suitable to alter the other characteristics of agas discharge lamp, such as colour temperature or light output (valuedluminous flux, lumen/watt).

This can now be achieved by operating a gas discharge lamp periodicallywith two dissimilar, substantially different current intensities. On theone hand a high current value is applied to the lamp for a certainperiod and on the other hand the lamp is allowed a recovery time with asubstantially lower holding current.

This cycle is repeated periodically so that averaged over time it isensured that the lamp remains in the conducting state and takes up thelamp nominal power rating P_(N). By changing the duration of therespective time intervals and the amplitude of the current pulses duringeach respective time interval the colour temperature and light output ofthe gas discharge lamps, in particular the Na-high pressure lamps, canbe changed. To date, however, this change requires that to achievedifferent current intensities for the main pulse and the substantiallylower holding current pulse the chokes arranged in the load circuit beswitched over by additional switches. These switches are expensive andcomplicated.

SUMMARY OF THE INVENTION

To solve the problem illustrated it is therefore proposed by theinvention to avoid switching the chokes and to provide only one chokewhich, however, is provided periodically via at least one switchingelement or element pair with voltage pulses, the frequency of thevoltage pulses being modulatable by means of a control signal. Thisachieves modulation of the effective reactance of the load circuitwhereby the load current, which also flows through the gas dischargelamp, is as it were also modulated.

A further independent solution of the explained problem is found in theuse of the circuits mentioned at the beginning for the pulsed-modeoperation of gas discharge lamps, preferably with a pulsation frequencyin the range of 50 Hz to 1000 Hz, whereby the gas discharge lamp isarranged in series with an inductance in a load circuit.

The idea on which the explained solution is based is that of controllingthe momentary operational overloading of the lamps and the holdingcurrent phase by way of a frequency change which involves alternativefrequency, stepwise or sine-like, i.e. a continuously, periodicallyformed frequency change. As the mean value the nominal power rating isreached so that the lamp is not thermally overloaded but the lightoutput and its colour temperature can be changed.

If necessary the holding current could also be dispensed with but it isexpedient to retain it so that the lamp does not need to be re-ignitedafter a main current pulse. It also allows a non-sine-like but similarlyconstituted, in any event continuing frequency change to avoid standingwaves in the lamp which lead to optical disturbances or to unpleasantflickering.

By the additional influence of a pulse width modulator a pulse widthmodulation can at the same time be introduced along with a distinctchange in the frequency within the scope of the frequency modulation.This allows a steady brightness variation of the lamp as it cansubstantially change the amplitudes of the current intensities bychanging the turn-on times.

An additional advantageous effect, with regard to the power loss, isthat when a high load current is to be switched the associated operatingfrequency of the inverter is low and the (high) frequency alternativethereto, with which the holding current flows into the gas dischargelamp, does not significantly load the switch. In this way a highoperating frequency can also be effected herewith without additionalmeasures.

When dimensioning the sole reactance it is advantageous, in the case ofthe stepped frequency change with alternative frequencies, to select aresidual ripple of the lamp load current i_(L) in which disturbingoptical waves (flickering) are just avoided. These limits lie, dependingon the lamp, between 10% to 30% and are thereby dependent upon therelevant current mean value or current effective value. Here too, theselected frequency alternation is surprisingly advantageous asparticularly with high frequency f2 and active small holding current asmall ripple is necessary, and this can also be ensured through the highfrequency. With reduced frequency f1 the load current increases, withwhich the main current pulse begins and the current ripple increases,and the current mean value or effective value also increases, whichagain ultimately ensures that the necessary limit values are maintained,despite a low frequency f1 with the same choke value L1.

The invented ballast is advantageously further developed in that themodulation form is varied in different manners, including pulse widthmodulation. The polarity reversal according to claim 9, with whichuniform loading of the electrodes of the gas discharge lamp is ensured,is particularly advantageous. Specific circuit configurations aredescribed herein. The association of the frequency values f1, f2 withthe respective current intensities of the current pulses is taught byclaim 9.

According to a specific feature of the invention, a realisation of thecontinuing, periodic frequency alternation is obtained with an inverterof which the modulation depth can correspond to the amplitude differencebetween the current main pulse and holding current pulse.

According to further specific features of the invention, it is possibleto obtain a lamp-friendly ignition possibility which is ensured by theaction of a control circuit with which a high effective voltage can beapplied to the gas discharge lamp for a selectable ignition period TRtypical of the lamp.

According to another aspect of the invention a self- or externallycontrolled half or full bridge circuit, as described herein, is used toprovide two astable operation points, the one providing a main currentpulse and the other providing a holding current pulse which are,averaged over time to provide a lamp-typical nominal power PN in the gasdischarge lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, exemplary embodiments illustrate specific forms of the describedinvention: in the Figures:

FIG. 1 is a current diagram of a first inverter circuit with possiblelamp types,

FIG. 2 is a current diagram of a second inverter circuit in a halfbridge circuit with the same lamp types as shown in FIG. 1,

FIG. 3 is a current diagram of a set interruptor circuit arrangementwith switchable frequency sources which control the switching elementfor generating the frequency modulated alternating voltage pulses,

FIG. 4 is a block diagram for a rectangular frequency modulation of theoutput signal of an inverter,

FIG. 5 is circuit diagram of an output branch of aself-excited--capacitive or inductive--fed back inverter having aseries-resonant load circuit and the possibility of frequency controlintervention,

FIG. 5a is a current diagram of an alternative load circuit to that ofFIG. 5,

FIGS. 6 to 8 are timing diagrams of current and voltage characteristicsfor operation with alternative frequencies,

FIG. 9 is a waveform which shows the current characteristic forcontinuous, periodic operation, and

FIGS. 10 and 11 show the ignition procedure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a typical appearance of a full bridge provided with fourswitching elements S1, S2, S3, S4, which feeds, from a d.c. voltageU_(dc) =udc, a lamp load circuit, comprising a choke L1 and a gasdischarge lamp GE, in its bridge branch. This d.c. voltage can beobtained from a battery but it can equally well be a rectified andsmoothed alternating voltage, e.g. 220 V/50 Hz mains supply voltage.Types of lamps that can be used are the gas discharge lamps shown in theright-hand part of the Figure, e.g. a high pressure or low pressure gasdischarge lamp. A direct, indirect or even non-heated lamp can equallywell be used. Depending on the use of one of the aforementioned lamptypes, an ignition capacitor C1 can be employed. This is to be connectedeither in parallel to the electrodes of the non-heated lamp GE or,insofar as a heated lamp is used, in the conventional type of circuit,in which the capacitor C1 connects in each case a terminal of theoppositely situated heating coils.

The bridge circuit is now operated in a manner such that the respectivediagonally opposed switches S1, S4 and S2, S3 are switched synchronouslywith one another so that a bipolar alternating voltage u_(ac) of a highand controllable frequency can be supplied to the load circuit in thebridge branch.

The half bridge circuit shown in FIG. 2, in which one pair of switchingelements is replaced by an alternating voltage type middle point (bymeans of two C/2 capacitances), operates in a comparable manner. Theremaining switch pair S1, S2 is hereby however not switched alternately:only one of the two switches is clocked for one selected currentdirection in the load circuit. If the load current iL is positive,clocking the switch S1 suffices; if the load current iL is negative onlythe switch S2 need be clocked. As in FIG. 1, free running diodes areobviously provided which are each provided in parallel to the switchingelements but are not illustrated.

FIG. 3 shows an interruptor which acts on the load circuit L1 GE withvoltage pulses from the d.c. voltage U_(dc), denoted above, via a switchS1 and hereby applies a load current iL into the lamp GE. As in thepreceding Figures only one choke L1 is provided. The GE lamp can also beconnected in different ways with ignition or smoothing capacitors C1, onthe one hand parallel to a non-heated gas discharge lamp or on the otherhand in connection with two opposed heating coils. Smoothing or ignitionoccurs via the above-mentioned capacitor C1.

Furthermore, a control for the switching element S1 is illustratedschematically in FIG. 3, and such control can likewise be used in thepreceding Figures. With a modulation frequency f respective ones of twofixed frequencies f1, f2 are selected and supplied, by way of suitablecontrol means, if necessary floating or displaced in potential, to theswitching element S1. The two fixed frequencies f1, f2 are thefrequencies that are required for a main pulse and a holding currentpulse. The frequency f1 is thus selected to have a value of the order of20 kHz; this represents the low frequency with which the main currentpulse is supplied to the load circuit. Its current amplitude amounts tobetween 1 A and 50 A. After switching over by way of the modulationfrequency f, the higher frequency f2 is supplied to the switchingelement S1. This results in the holding current pulse that is lower incurrent value. With a frequency between 60 kHz to 200 kHz it lies in acurrent value range of 50 mA to 500 mA. The modulation frequency f ishereby selected so that, averaged over time, the current values as maincurrent pulse and holding current pulse supply just the nominal powerrating to the GE lamp, although the main current pulse supplies ittemporarily with a substantially higher power than the nominal power.This achieves the effect that the high pressure lamps can have colourtemperature and light output adjusted and can be improved, but at thesame time their nominal load is not exceeded. The holding current whichis also referred to as "keep alive current" ensures that the lampremains ionised, i.e. conducting, during the recovery period of the lampand that re-ignition need not take place before the main current pulseis re-applied.

Hitherto, the realisation of the switching elements S1, . . . S4 hasremained unmentioned; they are as a rule formed of bipolar transistors,however MOS-FET transistors or RET transistors can equally well be used.

Like the control circuit explained with reference to FIG. 3, FIG. 4 nowshows a similarly designed control circuit. It can equally well be usedin the circuit examples shown in FIGS. 1 to 3. The frequencies f1, f2are here switched over directly by way of a frequency controllableoscillator 10 to which the control signal f--which determines thefrequency--is supplied. An oscillator of this kind can be a VCO; thelevel of the voltage f determines the output frequency f1, f2 of theoscillator 10. The form of the frequency change, i.e. the frequencymodulation can also be changed more simply, e.g. can take on asine-like, triangular-like or any other suitable form; only the voltageform of f is adapted. In FIG. 4, in the left-hand part of the Figure, arectangular characteristic of the frequency modulation f is shown by wayof example. It is comparable to the switching of the fixed frequenciesshown in FIG. 3 as it operates so that alternatively either thefrequency f1 or the frequency f2 is supplied from the oscillator 10 tothe converter 20 that is provided as one of the conversion circuit typesdescribed above. The converter 20 then supplies the voltage pulse u_(ac)to the load circuit L1, GE and gives rise to a current i_(L) which is tobe pulsed.

The oscillator 10 is supplemented in FIG. 4 by a monostable flip flopwhich prevents a frequency alternation or a frequency modulation by wayof the input signal f for a predetermined period TR at the turn-on timepoint. This prevention causes a predetermined fixed frequency, forexample only frequency f2, or approximately the load circuit resonancefrequency f0, to be supplied to the inverter during the ignition periodTR. The ignition period may be in the range of a few milliseconds toseconds and enables sufficient ionisation to build up in the lamp andignite it gently. Depending on the type of lamp the ignition period TRcan be varied depending on whether an ignition-friendly or less readilyignited lamp is used.

In FIG. 5 a self-excited inverter is used to which are connected acoupling capacitor C0, a choke L1 and a gas discharge lamp GE, and asmoothing capacitor C1 in parallel with the latter. The thus formedseries resonant load circuit LK has a resonance frequency f0 of theorder of magnitude of 1 kHz to 60 kHz which is determined by L1 and C1.It is fed from the output branch--here merely outlined--of theself-commutated inverter (resonance converter) which in turn draws theutilised energy from the d.c. voltage supply Udc. An adjustable emitterresistance is provided in the emitter circuit of the lowest of the powertransistors connected in series.

In FIG. 5a an alternative load circuit is shown in which the resonancefrequency f0 is likewise determined by L1 and C1 when L2<<L1.

The adjustment of the emitter resistance incorporated in this way allowsthe desired frequency modulation which, with a series resonant loadcircuit, leads to an amplitude modulation of the current iL flowing inthe load circuit. The adjustment--also modulation--of the emitterresistance can also be effected by connecting in parallel other emitterresistances or by bridging the emitter resistance or by adjustment bymeans of a control element (MOS-FET Transistor T) connected in parallel.An adjustment of the emitter resistance effects an earlier or latersaturation of an inductive coupling element (not shown here) whichconnects the load current circuit to the control of the transistors ofthe output circuit. Its saturation then changes the frequency of theself-excited inverter, whereby the goal of modulating the outputfrequency of the alternating voltage pulses of the inverter is achieved.Apart from the mentioned possibility of changing the emitter resistanceof an output transistor of the output branch, the frequency modulationcan also be effected by loading an auxiliary winding or a controlwinding of the inductively coupling transformer which ensuresoscillation.

Instead of a self-commutated inverter, an externally commutated invertercan obviously also be used, for example as shown in FIG. 1 or FIG. 2.The voltage frequency modulation effects a current amplitude modulation.This modulation can occur with a predetermined modulation depth whichthen corresponds to the main current pulse and the holding current pulsein FIGS. 1 to 3. The amplitude modulation occurs with a frequency offrom 50 Hz to 1000 Hz so that it remains invisible to the eye. It ishereby advantageous to tune through a plurality of frequenciescontinuously, so that a specific frequency--as in the stepped frequencyalternation--does not exist and standing waves which would lead tooptical waves and cause the lamp to flicker are avoided exceptionallywell.

The pulsed-mode operation shown in FIG. 5 is thus achieved by pulsatingthe envelope curve of the output current with a frequency of 50 Hz to1000 Hz. The amplitude modulation, i.e. the modulation depth, lies inthe range of 1:10 to 1:1000.

Ignition of the lamp occurs either by raising the frequency to near theresonance frequency f0, by selectively coupling high voltage pulses orby applying a high ignition voltage to the gas discharge lamp GE for alonger period. In addition a change in pulse duty factor (pulse widthmodulation) enables the control and adjustment of the output power, i.e.the brightness. Such a brightness variation can equally well be effectedby changing the frequency.

In an experiment with a circuit shown in FIG. 5, the waveform shown inFIG. 9 was obtained. Modulation of the frequency occurs in a range of 20kHz to 70 kHz, and the resonance frequency of the load circuit lies atabout 30 kHz and modulation occurs between the above-mentionedfrequencies during a period of 20 mec to 1 mec (i.e. between 50 Hz to 1kHz).

FIGS. 6 to 11 show current and voltage characteristics that are obtainedwith the converters 20 and the control parts 10 just described.

FIG. 6 shows clearly the pulsed-mode operation of the gas discharge lampby a current pulse of high amplitude with the frequency f1 which lies inthe order of magnitude of 20 kHz. Here the lamp is supplied with powerthat lies considerably above its nominal rating however for only a shortinterval T1. The frequency is then adjusted to the substantially higherfrequency f2, in the example about 120 kHz. This frequency alternationcauses the load current to fall to the now low holding current valueduring the duration T2. The two durations T1 and T2 lead to a periodduration T which lies in the order of magnitude of 200 Hz to 500 Hz.Thereafter, as shown in FIG. 6, a main current pulse follows again whichhas the above-mentioned frequency f1 during the duration T1. Such acurrent characteristic as shown in FIG. 6 can be obtained with a controlcircuit shown in FIG. 4 and with the control signal characteristic fshown in the left part of the Figure. The ratio of the currentintensities lies in the range of 1:100 to 1:1000. Their amplitudes canbe changed by both frequency variation f1, f2 and by pulse widthmodulation of a pulse width modulator as shown in FIG. 3. In any casethe lamp remains in a conducting state and, averaged over time, is notoperated above its nominal power.

FIG. 7 shows the diagram, corresponding to FIG. 6, of the alternatingcurrent characteristic at the output of the inverter and at the input ofthe load circuit L1, GE. This again makes the frequency alternationapparent, which is shown only schematically.

The frequencies and frequency ratios shown do not correspond in size tothe actually obtained relation. They merely show clearly the change ofthe frequency and the thereby achieved pulsating operation of the GElamp.

FIG. 8 shows a bipolar current characteristic, attainable with thecircuits shown in FIG. 1 or FIG. 2, which helps to spare the lamp andenables uniform utilization of the electrodes. Here--as shown in FIG.6--a main current pulse and a holding current pulse are applied withdistinctly different current amplitudes, however the sign of the currenti_(L) is inverted alternately, i.e. in each second period T. This isachieved, for example, with the circuit shown in FIG. 2 in that theclocking of the switching element S1 is discontinued, whereby the directcurrent i_(L) decreases to zero. The clocking of the switching elementS2 is then begun whereby the direct current i_(L) increases in theinverse direction. Both manners of operation are possible; a holdingcurrent pulse can be converted--its sign inverted--to a main currentpulse, and holding current pulses and main current pulses, which in eachcase have different current directions, can equally well be convertedinto one another.

FIG. 9 shows the current characteristic which is obtained with a circuitarrangement shown in FIG. 5, here in particular by sine-like modulationof the frequency by way of the likewise sine-like adjustment of theemitter resistor via a control transistor T; it leads to the amplitudemodulation, shown by f3 in FIG. 9, in the series resonant load circuitin FIG. 5. Here too the principle applies that with a high current thereis a lower frequency value f1 and with a lower current i_(L), a higherfrequency value f2. Again the advantageous effect exists that with highcurrents lower operating frequencies and with low currents highoperating frequencies, with which the power loss of the output of theself-excited oscillator is minor.

Unlike in FIG. 6 or FIG. 8, in this case no direct current is clockedbut an original alternating voltage is applied into the load circuit,the increase of its frequency effects a reduction in the wave amplitudeand thus a lower effective value. The statements with regard to thecurrent amplitude values for FIG. 6 and FIG. 8 apply here analogouslyfor the effective value.

Finally, FIGS. 10 and 11 show the effect of the ignition circuit 11which allows a high alternating voltage to be applied to the lamp GEfrom the converter 20 for a predetermined period TR. When switching on,the monostable flip flop 11 first suppresses the steadily providedfrequency modulation or frequency alternation f1, f2, by holding theoutput frequency of the converter 20 and of the oscillator 10 to apredetermined--low--value. This value lies near the resonance frequencyf0 with which the still undamped--the lamp has not yetignited--resonance circuit supplies high ignition voltage.

After successful ignition the effect of the monostable flip flop 11 onthe oscillator 10 is removed so that the steady operation, i.e. themodulation and pulsation of the current can be utilized. The ignitionachieved in this way corresponds to a cold start. If desired hot startcan be provided by utilizing the ignition capacitor C1 as shown in FIG.1.

If the pulsed-mode operation of gas discharge lamps allows a change intheir characteristics and an improvement in their light output, this isachieved according to the exemplary embodiments of FIG. 3 andappropriate control of the device shown in FIGS. 1 and 2, by retaining asingle choke and by an alternating astable exchange between twofrequencies. This frequency modulation leads to a change in theeffective reactance f.L1, which gives rise to a current main pulse--withlow frequency--and a holding current pulse--with high frequency f2. Therespective amplitudes and intervals T1, T2 of the pulses are tuned sothat averaged over time T the lamp is supplied with the power P_(N). Thedirect change of the lamp current by frequency modulation of the voltageu_(ac) corresponds, with an inverter as shown in FIG. 5 or FIGS. 1 or 2(with appropriate control), to the indirectly obtained amplitudemodulation which is effected by frequency modulation of an alternatingvoltage signal that is supplied to the load circuit (L1, GE). Thisamplitude modulation or amplitude modulation-like envelope curve f3reproduces the pulsed-mode operation described above. Its maximacorrespond to the main current pulses, its minima correspond to theholding current pulses. If the type of frequency modulation is varied,e.g. from a sine-like modulation to a rectangular modulation, e.g. bychanging the emitter resistor stepwise--the envelope curve f3 is alsoadapted correspondingly. This can extend from a sine form to arectangular form.

In the case of frequency alternative operation the ripple contentremaining in the load current due to the smoothing effect of the chokedeserves particular attention. It must not exceed a predeterminedthreshold value that is dependent upon the level of the respectiveactive current pulse. When the frequency is increased the currentdecreases, at the same time the higher frequency has the effect thatcontrol precision is sufficient, i.e. the limits of the ripple contentare not exceeded. The ripple amplitude thus in relation to the totalcurrent remains approximately constant or lies in the tolerable range.Disturbing acoustic and optical waves in the lamp cannot arise or becaused; the light effect remains uniform to the observer.

The selection of choke value L1 is likewise of importance since itdetermines the ripple content that must not exceed particular limits.Aside from determining the choke value, however, adaptation to apredetermined choke value by changing the fixed frequencies f1, f2--asin FIG. 3--can also be achieved, whereby the level of the current pulsescan be adjusted relatively freely and over an extensive range. Thus thechoke and said frequencies can be selected in a such manner that agreater or lesser separation between the permitted ripple amplitude andthe critical value is provided.

The smooth shifting of the control frequency towards or away from theresonance frequency f0 for igniting and brightness control purposes,already mentioned above, will not be described in more detail. However,it can, for specific applications, indeed be added supplementarily tothe circuit arrangements for pulsed-mode operation of lamps describedherein, within the scope of the invention. This depends on theparticular application.

We claim:
 1. A ballast for the pulsed-mode operation of gas dischargelamps having a buffered feed d.c. voltage from which the utilized powerfor the lamps can be derived, said ballast comprising a reactance,acircuit for connecting said reactance in series with a lamp, and atleast one electronic switching device connected to supply the reactanceand the lamp periodically with voltage pulses of first and seconddifferent frequencies, respectively, and which are, frequency modulatedin response to a control signal, the switching device including at leastone switching element connected as an interrupter and further includinga free running diode connected to take up load current of the loadcircuit comprising the lamp and the reactance when the switching elementis non-conductive.
 2. A ballast according to claim 1, characterized inthat the switching device is arranged to effect the frequency modulationwith alternative frequencies.
 3. A ballast according to claim 2characterized in that said switching device is constructed such that afirst frequency in the range of 10-30 kHz is supplied to the loadcircuit via at least one switching element for a predetermined firstinterval, and for a predetermined second interval second frequency inthe range of 50-120 kHz, is supplied, and such that the frequencyalternation occurs with an alternation frequency that lies in the rangeof 200 Hz-500 Hz.
 4. A ballast according to claim 3 characterized inthat a series capacitor is included in the load circuit.
 5. A ballastaccording to claim 1 characterized in that the switching device includesa pulse width modulator connected to influence said control signalwhereby the voltage pulses are frequency modulated.
 6. A ballastaccording to claim 3 characterized in that said switching device isarranged to supply voltage pulses of different frequencies stepwise to aload circuit comprising said lamp and said reactance such that during afirst predetermined interval a current pulse of a first predeterminedfrequency and in the range of 1-50 Amp. is supplied to said loadcircuit, and during a second interval a lesser current pulse of a secondpredetermined frequency and in the range of 50-500 Milliamp., issupplied to the load circuit.
 7. A ballast according to claim 6,characterized in that said switching device is constructed and arrangedto permit adjustment of the amplitudes of the respective current pulsesto the characteristics of the lamp and of the selected reactance byadjustment of the frequencies of the pulses.
 8. A ballast according toclaim 6 characterized in that said switching device is constructed andarranged to change the amplitudes of the respective current pulses byvarying the pulse duty factor.
 9. A ballast according to claim 6,characterized in that said switching device is constructed and arrangedsuch that the total power, comprising said current pulses supplied tothe lamp during one period, does not exceed the nominal power rating ofsaid lamp.
 10. A ballast according to claim 2, characterized in thatsaid ballast includes a converter in the load circuit comprising saidlamp and said reactance and arranged to produce continuous periodicfrequency modulation, and wherein said switching device is constructedand arranged such that said frequency modulation produces an amplitudemodulation of the alternating voltage in the range of 50 Hz to 1 kHz ofthe alternating voltage supplied to the lamp and the modulation ratiolies in the range of 1:10 to 1:1000.
 11. A ballast according to claim10, characterized in that the modulation depth corresponds substantiallyto the amplitude difference between successive pulses of differentfrequency.
 12. A ballast according to claim 2, characterized in thatsaid switching device includes a control circuit which suppresses thefrequency alternation for a predetermined lamp-type-dependent period andonly after the predetermined period allows the alternation of thevoltage pulses.
 13. A ballast according to claim 12, characterized inthat the control circuit is arranged to intervene in the ignition of thelamp in the manner of a monostable flip flop, but does not influenceregular operation of the lamp, and each initial intervention gives rise,for a predetermined period of time, to a frequency for the voltagepulses which lies near the resonance frequency of the load circuit whenthe lamp is not ignited.
 14. A ballast according to claim 2, wherein theswitching device includes a pulse width modulator connected to influencesaid control signal whereby the voltage pulses are frequency modulated.15. A ballast according to claim 2 wherein a series capacitor isincluded in the load circuit.
 16. A ballast according to claim 1 whereinsaid switching device is arranged to supply voltage pulses of differentfrequencies stepwise to a load circuit comprising said lamp and saidreactance such that during a first predetermined interval a currentpulse of a first predetermined frequency and in the range of 1-50 Amp.is supplied to said load circuit, and during a second interval a lessercurrent pulse, of a second predetermined frequency and in the range of50-500 Milliamp., is supplied to the load circuit.
 17. A ballastaccording to claim 16, wherein said switching device is constructed andarranged to cause the current pulses, during a first period whichcorresponds to the total of the first and second intervals, to have apositive polarity and during a subsequent period to have a negativepolarity.
 18. A ballast according to claim 1, wherein said switchingdevice is constructed and arranged to cause the current pulses, during afirst period which corresponds to the total of the first and secondintervals, to have a positive polarity and during a subsequent period tohave a negative polarity.
 19. A ballast according to claim 16, whereinsaid switching device is constructed and arranged to change theamplitudes of the respective current pulses by varying the pulse dutyfactor.
 20. A ballast according to claim 1 characterized in that saidswitching device is constructed and arranged to cause the currentpulses, during a first period which corresponds to the total of thefirst and second intervals, to have a positive polarity and during asubsequent period to have a negative polarity.
 21. A ballast accordingto claim 20, wherein said switching device is constructed and arrangedto change the amplitudes of the respective current pulses by varying thepulse duty factor.
 22. A ballast according to claim 17, wherein saidswitching device is constructed and arranged to change the amplitudes ofthe respective current pulses by varying the pulse duty factor.
 23. Aballast for the pulsed-mode operation of gas discharge lamps having abuffered feed d.c. voltage from which the utilized power for the lampscan be derived, said ballast comprising a reactance,a circuit forconnecting said reactance in series with a lamp, and a plurality ofelectronic switching devices connected to supply the reactance and thelamp periodically with voltage pulses of first and second differentfrequencies, respectively, and which are frequency modulated in responseto a control signal, said plurality of switching devices being connectedin a bridge circuit, with first and second pairs of said switchingdevices each comprising two series-connected power semiconductors, saidpairs each being connected across a feed d.c. voltage source, and a loadcircuit comprising said lamp and said reactance being arranged in onebranch of the bridge circuit.